1. Field of the Invention
The present invention relates to a signal processing apparatus and method, and a program. More specifically, the present invention relates to a signal processing apparatus and method, and a program which provide easy and reliable noise reduction using a plurality of filters.
2. Description of the Related Art
Mosquito noise and block noise are examples of noise caused by image compression processes using Moving Picture Experts Group (MPEG) techniques.
Mosquito noise is noise which produces a pseudo outline along the outline of an object on an image. Block noise is produced in units of discrete cosine transform (DCT) blocks in compression processing. Block noise causes discontinuities at boundaries between DCT blocks.
Mosquito noise and block noise are believed to be caused by the lost of high-order image information, or high-frequency components, due to the quantization in DCT transform in image compression processing.
Algorithms for reducing such noise have been proposed. For example, a method for reducing mosquito noise is disclosed in, for example, Japanese Unexamined Patent Application Publication No. 2004-336651. In this method, an edge region in an image is detected and mosquito noise is removed from the detected edge region, while the sharpness of the image is maintained, using a non-linear smoothing filter.
A method for reducing block noise is disclosed in, for example, International Publication No. WO 98/54892, in which a parameter necessary for the determination of block distortion from input image data is calculated and block distortion is determined based on the calculated parameter and a detected parameter which indicates the level of difficulty in image coding. In this method, a result of the determination of block distortion is used to calculate a correction value for reducing the block distortion, and the input image data is corrected using the correction value.
However, one of a mosquito noise reduction technique and a block noise reduction technique would not reduce both mosquito noise and block noise. A simple combination of both techniques may cause a conflict between the effect of mosquito noise reduction and the effect of block noise reduction or may cause a result of one of the processes to adversely affect the accuracy of the other process.
Specifically, filtering processes applied to a given region using a plurality of filters may halve effects. Or, a filtering process applied to a region using a filter may prevent the region from being detected as a region to which a filtering process is to be applied using another filter.
Accordingly, methods for reducing noise using a combination of mosquito noise reduction and block noise reduction have been proposed (see, for example, Japanese Unexamined Patent Application Publication No. 2005-318614). In the method disclosed in Japanese Unexamined Patent Application Publication No. 2005-318614, an input image is divided into a plurality of blocks based on a variance image of the input image, and the obtained blocks are classified as being either smooth, texture, or edge blocks. The smooth blocks are filtered for block noise reduction, and the edge blocks are filtered using a fuzzy filter for mosquito noise reduction. Therefore, effective noise reduction of the input image is achieved.